He doesn't. Subtle movements and noticing them requires an attention to detail Rodimus usually lacks.

"If he wasn't I wouldn't be coming to you," he admits. No, if Drift had died he'd have gone straight to Tarn, regardless of what the cost to his own welfare would be. Rodimus doesn't do things by halves - he kind of figures Megatron knows that by now. "He's with Ratchet. Oh, right - who Tarn decided didn't need both his legs."