There are two contexts in which this question can be posed : (1) in your own case and (2) in the case of an agent without experience. 'Unknown' can mean 'unknown that it is' or 'known what it is'. I take it that both senses are implied.
1 If 'a priori' means 'prior to or independent of experience', then (I think) you cannot properly ask the question concerning yourself since you already have experience, and specifically experience of coming to know objects previously unknown in your experience. Inductively, but not a priori, this gives you good grounds for supposing that if you have encountered previously unknown objects in the past then you will also encounter them in future.
2 In the case of someone who has no experience of knowing any objects, I don't see how this person could have any idea of the probability or otherwise of there being unknown objects since no known object has been encountered. 'Unknown object' contrasts with 'known object' but ex hypothesi for this person there are no known objects to contrast with.
3 I think there is a general problem with a priori probability if we take a priori in anything like its standard sense (as above). Probability - likelihood - is probability on the evidence or given certain data :
P (A|B) - the probability of event A, given event B. But a priori, no events are or can be given or any relevant experience. So much is contained in the very idea of the a priori. So while it is interesting to speculate on the probability of there being unknown events, I don't think the a priori is the standpoint from which to approach the question.