Honestly it isn't that far stretched that guards notice a pale, muscular man that they have never seen before dressed up as someone they saw go into the bathroom and never come out. This has always irked me about the entire series, Hitman 2/Contracts sort of had the right idea, Blood Money made disguising a boring way of playing the game.
If disguises worked like they did in Blood Money, Absolution would be one the easiest stealth games of all time.
It's really fundamentally broken though. Keep playing and you'll make this discovery soon enough. It's shaped the game from "decipher the pros/cons of each disguise, work out how to get them, in what order, and how you can manipulate the AI and environment to your favour all the while you stay in plain sight" into "I hope you have instinct! Or can find the magic nobody-detects-you outfit!". The trial-and-error tedium it introduces on higher difficulties is absurd.
Like, the idea that people
can spot you while wearing a similar outfit to them is okay to me, if it's contextually relevant and mechanically sound. Squad leaders who know their troops. Screwing around and behaving like an idiot making you seem suss and out of place. But not like the way Absolution does it. Like my early example: chefs in King of Chinatown will identify you in a chef outfit regardless of any logic or reason.
I really, truly feel Absolution is total utter shit in the disguise department. To the point where I derive no enjoyment from it what-so-ever.
EDIT: I guess my cliff notes would be that I feel stealth in Absolution has almost nothing to do with cerebral level puzzle solving of disguises and social stealth, and almost everything to do with mechanical based hide-and-seek, take downs, and instinct. Which means sneaking around reminds me more of Splinter Cell than Hitman.