The engine might be trying to play a specific sound, that is either from the map, or from the mod. It continues to do so, and the usage of memory will stack. I am just talking with experience, by looking in the console, with the Task Manager up, while looking at the RAM usage for hl2.exe, slowly but steadily, increases.
I've experienced this on the Trade server, as the map plays music, which I don't hear, because hl2.exe fails to load the music, on my end.