Not sure how the thieves steal electricity and cable TV cables. But 20 years ago, working in a telephone company as an Exchange technician, cable theft was a common news here when the price of aluminium and copper was very high.
Our Exchange has 40,000 lines capacity and there were 2 cables which had been cut twice. The cable was very long and 2/3 of it was underground before crossing the river. After the cable crossed the river using submarine cable, the remaining 1/3 length was overhead as it was only catering about 800 subscribers. The thief cut the portion which submerged from the river at the ferry point.
So our engineers designed a simple circuit out of op-amps or comparators. They used one of the pairs from the cable and short circuit it at the end of the cable. The circuit was able to detect 10 cable loops with 10 LEDs each indicating one cable. And the box was installed in our guide house as they work 24 hours in shifts. If the cable was cut (an open circuit was detected), one of the LEDs would light up and the security guard would alert our special team as well as the police.
This problem was later solved when fibre-optic cables were introduced into out cable network. There isn't much resale value for the glass in the cable.
Allen