Not had a bike stolen - but I should have on at least 3 occasions.
Ist time I arrived back at the bike 4 or 5 hours later and patted myself looking for which pocket I had (or hadn't as it turned out) put the keys. Thai guy standing outside a hotel across the road clapped his hands to get my attention and pointed to the bike - sure enough there were the keys in the ignition just waiting for someone to "borrow" it for the night. The hotel worker had obviously noticed them there beforehand (or hotel guest had noticed them and brought it to his attention) and was keeping an eye on it.
2nd time I had a flat tyre about 4am in the morning. Pushed the bike a few hundred metres to Soi Bukhao just off Pattaya Klang. Left the bike parked outside a 7-11 and jumped on a baht bus and back to Jomtien to sleep. 2pm the next day when awoke I couldn't find the keys. By 4 pm I decided they weren't on the premises so returned to the bike. 7-11 staff didn't have any keys handed in so I was forlornly standing at the bike - flat tyre, front wheel in lock position and no keys. A motor bike taxi guy came up and asked me if that was my bike. He told me to wait, and disappeared. Returning a few minutes later with another motor bike taxi guy. They went to their little kiosk and retrieved the keys. One of them had found the keys left in the ignition early in the morning. They put a new tube in the tyre - 150 baht - and were happy with a cold Chang from 7-11 as a tip.
3rd time - left the bike outside a hairdressing salon about 7 or 8 p, in Soi Lengkee. Returned about 2am - no freakin keys (again). Bussed it back home to Jomtien for the night. Back to the rental shop the next day to tell them my sad (stupid) story. One of the guys got a big ring of keys (they didn't know which of the spare keys was for which of their bikes) and I jumped on the back of his bike and into Pattaya. While going through trying each key in the ignition the owner of the hairdressing salon arrived to open up for the day - she smiled - went into her shop and returned with my keys.
So I should have had three bikes stolen but on each occasion I was saved by the honesty of the Thai people who work in the immediate area who "took care" of the situation for me in my absence.