Definitely a crack!
We were getting several frames back with that same issue. It would happen due to the way the bottom of the seat post slot was drilled. The hole would be drilled first and then the slot was cut. Since the drill would produce microscopic ridges in the hole, the stress from rider's bodies landing hard on the seat after jumps and whatnot would start a crack propagating. We then started adding the brazed on washer to reinforce the hole but obviously some still fail. Others failed due to people using the wrong size seat post and tightening the seat clamp way too much to get it to hold.
The best way to repair the crack is as follows: Remove the seat post (already done - I have a keen sense of the obvious don't I?), thoroughly clean the inside of the seat tube - make sure all of the grease is removed at least one inch either side of the crack, strip the paint away from the crack - I would recommend at least a quarter-inch below the crack and above, get a 1/16" drill bit and stop-drill the very end of the crack, carefully heat the existing washer around the hole until the silver solder melts and remove it, carefully remove any remaining silver solder from the seat tube, find a piece of copper tube that will fit snugly in the ID of the seat tube at the crack and clamp the copper tube to the seat tube, have the crack TIG welded with ER80S-D2 filler rod - DO NOT USE 4130 WELD ROD, remove the copper backing tube and make sure the repair weld has penetrated all the way through. If any weld bead is protruding into the seat tube, it will have to be ground or reamed so the seat post will fit and slide up and down, find a large stainless steel fender washer and round it so it fits snugly where the old washer was and carefully braze it to the seat tube, re-cut the slot and sand all of the edges with fine sand paper, prime and paint the affected area.
That's all there is to it! A job better left to a pro if you want it done right so it will last.
Good luck
Scott