Weird beeping when in gear

Hello all,new member here. My wife is 200 miles away and calls and says when she got in the 2000 Silo,she started it up then put it in drive and there is a loud alarm going off in the rear of the van on the passenger side. It does it in reverse and drive but not in park.She is afraid to drive it home tonight. Is there anything to worry about? I don't think the van has any kind of backup assist as there have never been any beeps or indicators in the 5 years we've owned it.
Thanks for any help.
John

Well she got it home. I went out with the flashlight last night and lo and behold after putting in gear or even neutral there was a very audible alarm in the rear of the van on the right side. Pulled the little storage compartment out and there is a little green alarm velcroed to the right rear quarter. Unplugged for now. All I can think of is its some kind of sliding door alarm. Will look later when I get home from work,but any thoughts or past experience?

That was my first thought....that the sliding door, either was not fully closed, or the circuit indicating a closed door was at fault......seen a lot of problems down at the bottom of the sliding door where the electrical "fingers" that mate with the contacts of the "B" pillar become corroded....sometimes sanding down the tips of the fingers with steel wool, and also the contacts, takes care of the problem.......

The sliding door system has self diagnostics, that can tell you if there is a code in the system.....not sure if the owner's manual goes into this or gives the procedure......

Well she got it home. I went out with the flashlight last night and lo and behold after putting in gear or even neutral there was a very audible alarm in the rear of the van on the right side. Pulled the little storage compartment out and there is a little green alarm velcroed to the right rear quarter. Unplugged for now. All I can think of is its some kind of sliding door alarm. Will look later when I get home from work,but any thoughts or past experience?
Reconnect that alarm and then hit the switch in the overhead that disables the power door. This will silence the alarm. But you still need to get it fixed. 200 grit sandpaper.