I had him outside on a leash.  Somehow he slipped out of his harness, and off he went!!  Thankfully, a neighbor was able to corral him and brought him back to me (Whew)

he slipped out of his harness!  yikes!!  glad you got him back..safe and sound......

So glad everything turned out okay! Our Holly is very slim, and every harness I've used on her she "can" get out of if she wants to.  She turns into some sort of "Houdini dog" and can be out in a second flat!   Most of the time (after 4 years) she would never do it now, but there are times that I don't trust her completely, like if we are going somewhere new.  Then I just put her collar on too, and I put a clip that connects both the harness and her collar "together" -- so it she were to slip out of the harness, she still can't get away,  because the collar is still on. I can make the collar snug enough for it not to go over her head, since I'm using the leash "on" the harness, and don't have to worry about her pulling on the collar.  

"She turns into some sort of "Houdini dog"

Funny you should say that.  The first thing I thought of after I got him back is that I was going to have to nickname him "Houdini", lol.