Basement floor drains fall into a rather gray area in the plumbing codes.

Many local codes require floor drains but there is no specific requirement for basement floor drains in the two major national model codes (IRC & UPC) however they are strongly recommended whenever practical.

The question then becomes, what is practical?

The elevation at which your house sewer enters the structure is determined by the depth of the municipal sewer main, the length of the setback from the municipal main and the required pitch of the sewer line. Obviously if your house sewer enters the basement at an elevation higher than the finished floor a basement floor drain connected to the house sanitary sewer system would not be practical.

In some regions when the house sewer enters the structure at an elevation which is higher than the basement floor they simply omit basement floor drains. In other jurisdictions they still require some form of basement drainage. The could come in the form of a dedicated floor drain piped to a sump pit while in other regions they simply accept the sump pit itself as a floor drain.

On the other hand, in some jurisdictions they say that a basement floor drain is primarily intended to discharge storm runoff water that may enter the basement and the codes strictly prohibit discharging storm water into a sanitary drainage system therefore the floor drains must discharge into a sump pit.

In all cases the codes strictly prohibit discharging a sump pump into a sanitary drain.

One key indicator as to whether your floor drain is connected to a sanitary sewer or a storm sewer is the presence of a P=trap under the floor drain opening. All fixtures connected to the sanitary sewer, with the singular exception of the water closet are required to have a P-trap. (Code prohibits installing a P-trap below a closet flange because a water closet has a built in trap in the bowl section, and installing a trap under a closet flange would be classified as "double trapping", which is expressly prohibited by code.

In some regions they require a "footer drain" to be installed around the perimeter of the structure and the footer drain is then connected to a storm sewer. When they have a footer drain connected to a storm sewer it is quite common to run a line from the floor drain to the footer drain pipe.

It is very easy to determine if your floor drains are connected to the sanitary drain. First, look around your basement,generally on the front side of the structure and see if you can find where your house "Main Drain" meets the "House Sewer", then passes through the footer wall. All structures are required to have a "main cleanout" at the point where the house drain meets the house sewer. For structures with a basement the main cleanout should be within 3' of the point where the sewer line enters the structure. If the house sewer exits the structure at an elevation which is above the finished floor you can be sure your floor drain does not connect to the house main drain because water simply does not flow uphill.

If you house sewer line is below the floor there is a good possibility that the floor drain is connected to the house sanitary sewer system, but you would still have to confirm that fact.

I would begin by pouring a large bucket of water in the floor drain, then visually examine your sump pit to see if the water ends up in the pit. If so, then you know the line is not connected to the house sanitary sewer.

If the water does not show up in the sump their is a strong likelihood that the drain does connect to the sanitary sewer system, but we must still confirm the fact. Fortunately there is a very simple test. Go to a Plumbing Supply House and ask them for the "peppermint oil" used to check drains. To conduct the test you first open the main cleanout, then pour the peppermint oil in the floor drain followed by a about a quart of water, then go to the main cleanout and smell the cleanout. If you smell a strong peppermint scent coming from the cleanout you know the floor drain is connected to the sanitary drainage system.
(If you do not have a commercial plumbing supply house nearby you can go to a larger grocery store and look in the section where they have all the flavorings for making bake goods, cookies and such or in the spice dept. You should be able to find a small 1/2oz bottle of peppermint oil. I must warn you ahead of time that the peppermint oil is a bit expensive. Perhaps $5 or $6 for a 1/2oz but when you figure the cost of cutting up the concrete to trace the line, it then becomes very cheap)

Obviously someone has had the problem of water back flowing out of that drain at some point in the past, otherwise they would not have attempted to cap the line in the manner you found. Rather than use that cap, which obviously is not working, you could go to a local hardware store and get a "DWV Test Plug". The test plug will look like a round rubber donut with a metal washer on each side and a bolt through the center of the washers. You slip the rubber donut into the drain opening, then tighten the nut on the bolt. As the nut tightens the two metal washers compress the rubber plug causing it to expand outwards and seal tight against the inside of the pipe. By capping in this manner you can effect a good water tight seal, but the plug could easily be removed if you have need of the floor drain.