Sensors perked, Optimus could catch enough of the voice to compare it to memory banks, and- unless this is a great coincidence, then it is her. He hasn't seen her for some time, but she's a human he recognizes, and moreso- one still identified as 'friendly'. And in this situation, it may be worth the risk to find out for sure.

And the most subtle way there?

He flashes his headlights, throwing the highbeams on for all of a second before turning them off- then flashign them again. The angle should make the light source visible to the human up ahead, unless she's truly distracted.

Then again, there's not much to distract her, unless she's seeing something he's missed.