I know this sounds nuts but Annabelle my 4 year old Schnauzer does the same thing that your 5 year old Yorkie is doing. She however usually "blows her poopy ball out" while she's sleeping and I honestly think its from gas. (flatulent) I think her belly builds up with gas while she's sleeping and POP a little poopy ball shoots out her butt.
She has only ever done this while sleeping (glad she doesn't sleep with me) and it only happens now and then. She has been to the vet and they say nothing is wrong with her. Oh, and there small little poopy balls too, strange deal!
So I've just come to the conclusion she gets built up gas in her tummy and it builds up till she blows, LOL
So far Schatzie my 9 month old Yorkie does not do this.