Meaning of 思えば in 誰かと思えば


The context of the phrase is that two have not met in a long time just meet up
and are talking to each other, the conversation goes like this:

Person 1 (The one who started the conversation): 久しぶりだな

Person 2: おやおや、 **誰かと思えば**

In the sentence "誰かと思えば" I know that 思えば is on the conditional form, but
there's no condition, so what is the meaning of the ば form on this sentence?

