What is odd about the blobs is that they are appearing out of sequence.....there on dsc00505 and dsc00570....not there on dsc00569 and dsc00582 .....so it appears to be comong and going.
It certainly looks like the sort of thing you would get with either condensation or a spot of rain......but even if you had a spot of rain on one frame..it dries...you would still get a residual bit of flare on the next frame, and yet here we have it not on 569 but it is on the next 570.
Of course it may have been spitting a bit on and off........but the blurred bit appears the same size and shape and in the same position on the occasions that it does appear.
TD trundles off to put on thinking cap again.
Aha...........just noticed your post, unkyUb, and Sue's mention of a smudge >>but that wouldn't tie into to some being ok and some having marks on them.....would it?<<< Yes, yes it could !!........Now the, were you using the lens hood ?
In the photos where the very bright sky predominates, the blob is there......where the sky does not predominate, such as the closer shots and the interiors, it is not there. Sooo,.............. I think what we have here is lens flare.
The smudge has been there all the time but the light has caught it and caused flare only under specific lighting circumstances....ie where the bright sky has predominated.
If that is the case, a good clean with a lens cloth is the thing
Don't use any of the liquid cleaners.....liquids can seep round the edges of the lens elements into the lens and onto the rear of the element....just rub in a circular motion with piece of fine, soft cotton fabric.