Custom Creatures can be any way you think is fun, for them being custom =P
It would be most consistent with the rest of PF if the demon activated the ability on a successful hit vs. AC OR on a special attack which only delivered the mark vs. touch.
I also think that on one hand saying "requires no action by the demon" and then having the demon "use an action to detonate" is kinda wierd. Have you also considered the sheer bookkeeping required? How about something that detonates immidiatly? Like some items that deal damage to someone attacking you. It kinda leaves a mark where contakt is made - actual contact, not a bare tangency - but then the mark bursts into flames / detonates a second later, so still in the same characters turn.