Always presume there is something lurking in a wall,water pipes gas pipes mains cable they could all be there lurking waiting to drop you in it.
Ther are a great many cheap detectors on the market that really are useless.
A good detector is accurate and capable of placing your drill hole to with in 15 mil,these detectors sell for around £50 and upwards,expect to spend nearer £100 for a real rolls royce.
I'm only going to use it occasionally you might say fair enough, drilling through a water pipe or gas pipe will cost you a lot more than that