In Japanese, the に particle can be used to mark past targets.1
Example: I am at the shops
おみせにいる
Info
In this example, おみせ (shop/store) is marked by the に particle marking it as the past target. It is the place that we were aiming to arrive at in the past.
いる is a verb telling us that the subject is existing. The place the subject is existing is within the past target marked by に.
This sentence has an invisible が subject, and as there is no sufficient context to suggest otherwise, it defaults to わたし (I).