identifying pipe in maintenance closet

I am trying to install a utility sink in the garage- the other side of the wall in the garage is the mainenance closet. In the maintenance closet, there is one 1.5" PVC pipe coming down from the ceiling into the concrete. There is also a 3" pipe. I know that the 3" pipe provide drainage to the sewer. However, I cannot figure out where the 1.5" pipe goes. I have turned all of the water on in the house and have not detected any water running down the 1.5" pipe. If this pipe provides drainage, it is more convenient to drain the utility sink into this pipe rather than the 3" pipes. Do you have any ideas the 1.5" pipe may go to?

The 1.5" pipe through the roof would be vent for something.

You could look below and see what it may be venting. You could tie into this pipe for the vent, buy may not be able to use it for a drain, if it's venting a lower fixture.

Terry,
The mainenance closet is on the first floor. I can only trace that the pipe goes from through the ceiling from the 1st to 2nd floor. Beyond that, I am not sure where it goes. If I understand correctly, since I would put a Y 6" above the concrete, there would be no fixtures below it and I could use it as a drain?

Thank you for all of your help!

vent

If it is a drain for an upstairs fixture, you could also not use it as a drain for your sink.

Are there any fixtures in your basement, such as a washing machine standpipe. The. 1.5 is probably a vent for whatever is down there. Code would not allow you to tie into this as a drain. It is hard to say without knowing more, but you could probably tie into the 3" for the drain and vent into the 1.5. However, if code compliance is not an issue, assuming you only intend to use the utility sink from time to time, you will not have a problem with just adding a p trap onto the 1.5.

The washer and dryer are on the third floor (but directly above the mainenance closet). There is a bathroom on the second floor directly above the maintenance closet. Could this be venting for one of those?

I am not heavily concerned about code compliance- I will remove the sink before I sell. However, I also do not want to do anything that will cause problems in the rest of the house.

"I am not heavily concerned about code compliance"

Homeowners never are concerned. That's why the US has compliance officers (plumbing inspectors) to keep everything on the up and up. When I went to third world countries, the plumbing was a shambles everywhere I looked.
Of course I couldn't drink the water there, the hotel rooms smelled, the harbor smelled when you boated through it and one hotel I stayed at couldn't rent out the first floor because the waste was backing up.

Since you know the 3" line is waste, cut a san tee into that for the utility sink.
You will need a vent and a p-trap.
If the 1.5" line is a vent, you can revent there.
If it's not a vent, you may be better off using something like a studor vent to keep the trap from siphoning and smelling up the room.

Point well taken Terry... I will add the san t to the 3" pipe. There is a cleanout about 3-4 feet off of the floor on the 3" pipe. Would code permit me to place the t in between the cleanout and the concrete?

Thank you for all of your help!