I have to confess to watching a stranger's bag, and asking my bag to be watched. But I can explain!
There was a huge snowstorm the day before, when people where trying to get home after New Year's. It was a madhouse, flights canceled, the airport was crowded and tempers short. After a mix-up with my original AND rescheduled flight that day (long story, but I got a voucher from the airline for their screw-up), I was scheduled on the first flight for the next day. Yay, I got to spend the night in the airport.
So those of us on that flight started lining up at the gate over two hours before. It was so early the coffee shop wasn't even open yet. The flight was delayed some more: a flight attendant was late. Naturally, after standing in line for a while, someone would have to answer the call of nature.
By mutual agreement, the person leaving wouldn't lose his place in line. He/she would ask his/her neighbor to watch his/her bag(s) while off to the restrooms. Said bags would mark that person's spot, and everyone near that bag kept an eye on it, not just the person asked.
Finally I couldn't wait any longer, I had to go! So I asked a person in line with me to watch my bag and I went to the ladies' room. Before I got there, I saw a flight attendant coming the other way. So I asked her if she was going to X City, and she said yes! Forgetting my needs (heck, I could use the lav on the plane!), I ran back to the gate and announced to the line that our flight attendant was here! Much happiness ensued.
This was before 9/11. I doubt anything like that would happen again.
ETA that was the ONLY time I watched a stranger's bag and asked a stranger to watch my bag.