Bulgaria has outgrown the days of brazen car thefts from the early 90's but you should still not let your guard down. A foreign-license plate might attract some unwanted attention.
Never leave anything of value in the car and do not leave bags or backpacks visible, even when empty.
For overnight street parking choose a well-lit location.
Do not park expenssive vehicles on the street.
Always double check that your vehicle is locked. Reports about the use of remote devices that prevent cars from locking are on the rise.
Call 112 in case you vehicle has been broken into or is missing.