Same here, running up stairs in hospital building in Electro once and also running in ground floor farm buildings in Novy Sobor twice now. No noise, zombie, enemy, under 5 other player servers, on stable 0.35 branch.
Tip: dont get attached to any character or gear!

Still happening to be on the 0.35, tends to be M4, M4 attachments or M4 magazines. It occasionally reappears and I can see it but will not allow me to pick it up, even after waiting 10-15mins trying both mouse wheel and tab! This occurs outdoors when on hill.