When I move out, the first thing I do is call the PoCo to end my service. I suspect the power was already "off."
The second possibility is that the bomb simply didn't work, and the sparky found it when he began troubleshooting the light. Bombs are not nearly as easy to make as "The Anarchists' Cookbook" leads you to believe. I've seen countless 'never-fail' recipes fail to perform.
For that matter, electric work isn't as simple as it seems. Anyone want to guess that the connected wires were of the same potential?
Look, the idea of landlords and tenants expressing malice towards each other are far from new. I recall one author who capitalized on this, by writing two books- one on how to 'tame' your landlord, and one on dealing with unwanted tenants.
I've also known folks who claimed to have done all manner of things in revenge- everything from removing the front door (to force a tenant out) to plastering fish inside the walls (to create odor).
Perhaps most distressing are the shelves of books at the local hardware store, purporting to replace a 5-year electrical apprenticeship with a $20 book, pretty pictures, and a few hours reading.
Then there's the matter of honest, ordinary wear and tear. I'm thinking here of the upscale New York building that actually blew up the other day. Gas leak? Unrepaired issues from a year ago? Non-permitted work, perhaps even unlicensed contractors? Fuggadaboudit.
Author Martin Cruz Smith, in the book "Red Square," described a wonderful bomb in great detail. Fans of Mythbusters will recall their attempts to re-create this bomb. It was a heck of a lot harder to accomplish, and far less effective, than the book led you to believe. That parallels my own experience, as well as actual law enforcement records: there are a lot more fizzles than real booms.
The only sabotage job I'm personally aware of occurred about eighteen-years ago during a brand new grocery store construction -- underslab.
The concrete contractor was so angry at the stub ups from the plumbers and electricians that he took to pounding more than a few over with a baby sledge hammer. (!)
In vino veritas.
He lets his handiwork be known to other tradesmen -- and of course, the missing stubs are missing.
It costs him his shirt, his reputation, and ultimately his license... as he went bankrupt. The GC back-charged him something silly as the slab had to be jack-hammered something frightful to recover the turned over stubs. There were plenty -- most still showing the effects of his hammer blows.
Apparently, this was his first pour for a grocery store and he figured that the slab would look like a clean flat sheet of concrete... whereas any modern grocer is going to have stub ups simply everywhere: so many chillers and freezers.
And then throw in the floor sinks for the plumber.
No wonder he was the low bidder.
The astonishing thing is that anyone could possibly pull such a stunt and not realize that the (forensic) evidence would point straight back to themselves?
BTW, another common sabotage against underslab pipes, concrete, can be defeated if caught early enough. Fresh concrete is quite vulnerable to acid -- even soda bottle acid -- as in Pepsi or Coke. (!) For this to work, you need to get right on it.
Concrete is extremely vulnerable to SUGAR. I don't know the chemistry, but even during WWII the French underground was encouraged to add sugar to German concrete fortifications -- the Atlantic Wall. It turns out that a tiny amount of sugar makes concrete stronger -- anything beyond tiny makes concrete weaker -- to the point of crumbling if hit by a hammer.
So when you douse newly poured concrete with soda pop -- any brand -- you're hitting it with both sugar and acid. The effect is not super rapid, but has been known to un-sabotage more than a few pipes.
So keep this in mind if any dribbles of concrete manage to get into your underslab runs. Quick action may save them.