I'd suggest giving some thought to how the magic actually works -- because figuring out a concrete magical effect will get you a long way towards answering the question "how does this escape the character's notice".
Just a few examples off the top of my head:
- The magic grants the protector great confidence and courage, just at the right moments.
- The magic forces weird coincidences in order to guarantee that the protector doesn't just strike off on their own.
- The magic causes the protector to fall in love with their charge, so the protection will feel like its of the protector's own volition.
What I'd really recommend is the you, the author, be able to point to one or two things and say, "Look, HERE, right here is where the magic is." That the magic's effect be specific, interventions that are clear and well-defined. Because the alternative is that the magic becomes vague, all-encompassing; a hand-wave saying "the magic made all this happen" without it being clear what the magic actually did, and then there really is no difference between the magic spell and authorial whim.
I'd also recommend you limit yourself to one or two effects -- they can repeat, but I'd recommend you keep them consistent. If the magic makes the protector dream of their charge every night and brings down torrents of rain in order to keep them snug in a tent, then that gives you twice as much to explain and justify; twice as much to walk the "this is magic but not too overt" balance with.
If you do this, you will have a promising answer in your hands. Because whatever magic effect you've chosen, is now both the one thing your characters need to dismiss as perfectly normal, and also a very specific, concrete effect you can actually highlight as being strange and curious.
Following the earlier examples:
- A person becoming uncharacteristically courageous, always to the benefit of one character, is precisely the kind of thing that might seem natural in the moment, but might raise some eyebrows if it keeps happening.
- The coincidences can strike everybody as very weird indeed; and it can take them a while to figure out how each coincidence winds up saving the charge's life.
- Falling in love with somebody can feel magical, unexpected, out-of-character, life-changing -- so maybe it is.
Note how each one of these gives you a different element to focus on, to play up as the center of this discovery. Each of them, obviously, also make for a very different story! (And you can come up with your own, tailored to the specific style and feel you're aiming for.)
But what each one does is gives you a focus; a specific thing to be taken for granted, but actually be strange and unusual. This can be an awful lot easier to do with a concrete effect -- bursts of courage; weird coincidences; a sudden romance -- than with the much more vague "compelled to protect them, for no apparent reason".
Hope this helps, and all the best!