TS Stands For Test Station

Every now and then someone will come along and spraypaint yellow "TS"'s on the sidewalks around the neighorhood with arrows next to them. The arrows they lead to little square metal covers with a hole in the middle.

From 99% Invisible, I figured it had something to do with the gas line since it was yellow, and that they was probably some sort of access point under the covers, but I couldn't figure out why they were using 'TS' instead something like 'GL' or 'GAS'.

Recently I found one of the TS's pointing to a more informative cover:

Apparently it's the test station for a Cathodic Protection System.