Depending on the context, you could call it a crush, particularly if you are talking about two adolescents.
A crush doesn't necessarily imply unbeknownst reciprocation; it's often one-way, and directed toward someone who is not in a position to share the attraction. Still, there would be nothing wrong with saying:
Dave and Jill both had a secret crush on each other.
I can't think of a noun with the inherent implication that neither is aware of the other's feelings, but I can understand your aversion to the word love, since love sometimes encompasses a devotion that goes beyond the initial stages of infatuation. Still, there are plenty of ways to convey the idea, without finding a single word for it:
- Unbeknowst to each other, Dave and Jill shared a strong attraction.
- Unbeknown to both, Dave and Jill were nursing a deep passion toward each other.
Unbeknownst and unbeknown both mean "without someone's knowledge." Perhaps that's the key adjective for the feelings you're trying to capture and express?