Sometimes you are in the middle of a recon for a physical assessment and you find a way in, right there on the spot. Do you exploit this vulnerability or just note it for later?

I say go for it.

This was the case a few weeks back. We were performing an on-site recon: we would walk the perimeter and try to go inside the target building and learn as much as we could from atmostpherics and their security measures in place.  
After about 40 minutes of roaming inside and outside the building, dressed as business people with a suit and a tie and a cellphone that never left our ears, we thought we had the site main security features, entrances, location of the targets and other details sketched, when I noticed that the service elevator on the far end of the 1st floor had the door open and no one was using it.