I got home yesterday and discovered the house server (ancient P100, HD falling apart, running PPP, masq & various proxies) had been reset by a power cut. On reboot it was having e2fsck problems (mental note: next reinstall will use ext3) and the DNS database was corrupted.
Just managed to bandage up the wounds and reboot the machine when we noticed the rather large amount of sand around the casing. It really was quite an odd thing to accumulate inside the house next to a computer. So we investigated further.
A huge ants nest. I guess that explains where the plague of flying ants inside the house had been coming from :/