I recently bought a bike online and I wanted to make sure it was the bike I paid for. I looked for the serial number on the bottom of the bike and couldn't find it. Where else would they put the serial number on a 24 inch Haro?
This image from the City of Portland police shows the common places for bicycle serial numbers. Note that they are usually stamped and aren't very visible.
Note that one area not shown on this image is the head-tube. That's where my latest bike had it. Otherwise, my bikes all have it in location #4 (bottom of the bottom bracket) or behind the seat post.
Note this posting from Bike Forum which shows the Haro serial number stamped on the bottom bracket.
There is also a registry of Haro bmx serial numbers, which might also help in identifying your bicycle:
This latter registry is particularly useful because it has user-submitted photographs of serial numbers. While the majority are on the bottom bracket, some are stamped very faintly into the rear dropouts.