Didn't you ask for a FLAME THROWER KIT ?
Seriously, whatever work shop that was, they MIGHT have forgot to take that out and put your sump plug back, probably they were flat out ................and totally forgot about it.
Every workshops make mistake everynow and then, just bring it back and tell them. If they regconise the issue, admit and appologise then its ok, otherwise if they don't then you can really tell how professinal they are and obviously there won't be a second time. Then you can start pm us the workshop's name .