Cache freshness timeout and cache access

Why is nix fetching 25MB source from if I do not activate a nix-shell environment for a long time? Why the internet access to cache? All required dependencies are in nix/store.

I always get:
[1/0/1 copied (0.1/155.7MiB), 7.4/24.9 MiB DL] fetching source from
Which takes 5 to 10 seconds.

I suspect there is some sort of timeout mechanism since this only happens if I do not use that shell env for a long time.