There are 9 species of coccidia that live in sheep's guts, and only one that really causes disease. If the vet has said keep an eye on them then it may be there was a low count, and if they are not unwell they may well be infected with one of the harmless ones. Sounds like your vet is very helpful, keep getting the advice and faecal samples done and you'll keep wormers effective in your wee flock for some time to come.