Is this a basement drain capped off?

[tperkcincy]

I have a small picture attached (hopefully not to small to read) but I have two of these in my basement and I think they are just drains that are capped off. It about 4" black PVC pipe cut flush to the concrete floor and about 5 inches deep in the hole is a cap that looks like it got threads on it but I tried to turn it and it won't move. I guess this is a drain but just in case I'd like to know if this could possible have a built in trap below for a shower? Also are these typically glued in or how do I open this up to look?

Thanks!

[markts30]

Sounds like a floor clean out...
Might be the body without a cap for a 3" cleanout - the threaded plug would go into a plastic or iron FIP (IBCO).
Is the threaded plug in the floor got any sort of pattern to the top of it? (Square raised or recessed lug... Slot?)
Can you take a photo of it...
If the cap appears to be rubber, it might be a test cap - they are attached with a "hose clamp" on the side....

[tperkcincy]

yes in the drawing I have the 4 lines are raised lines on the cap. Looks like it's made for you to grip and turn it to open it. However it won't budge.

[hj]

Raised ridges might mean it is a backwater valve. Plugs have recessed slots in them.