> ... any door ... wouldn't lock behind me. You are not alone with that precaution. Ages ago, before plastic gift cards were ubiquitous, I used to quest/urbex

> ... any door ... wouldn't lock behind me. You are not alone with that precaution. Ages ago, before plastic gift cards were ubiquitous, I used to quest/urbex with shims of thin aluminum trimmed to door jamb size (making them harder to detect). The buddy system also was used - "You stay on this side, I'll go through and test it can get opened from the other side." Extremely useful when entering campus/city steam tunnels that may not have another good door for hundreds of meters (the tunnels were secured to prevent access to the structures they connected).