I might be wrong, but I think if you drop the urn before 15:00 and when time hits exactly 15:00 urn have to disappear because it's spawning on the other side of map (so if nobody is carrying the urn and 5 minutes mark hits e.g. 15:00, 20:00 etc. urn will disappear OR it'll disappear and spawn on the other side of map). At least it's what I observed during my 470h in Deadlock.