Like many such scenarios, there is not really an adequate amount of information presented to make a rational decision.

IMO, unless there is some convincing reason for me to believe otherwise, I would assume the pack was left there by the owner on purpose and the owner is coming back to get it, and would just leave it be.

I might well take a picture of it, and record the location, but it's someone else's property and I am not going to mess with it, absent some very good reason to do so.