It would be incredibly difficult to notice Alastor's love for you.

To anyone aside from himself, it looks like he's merely your closest friend, but there is so much more that he feels for you. You would have to be looking very intently at him to notice his changes towards you. Although, he would never act like this towards you if he know anyone was looking at him that much.

The changes are so subtle, so unnoticeable.

When he looks at you, his eyes practically glow with sheer adoration and love. All directed towards you.

He was so incredibly lovesick, but he would only allow the tiniest amount of his affections towards you to show.

Oh, how his smile would grow just a tad larger and more genuine than usually when he was with you. You made him feel so much that it hurt.

Alastor loved you so much that he couldn't put it into words. He adored you with all his heart, and couldn't possibly imagine his life without you in it. To him, that would be hell. 

He'd be lost without you.

Alastor is much more relaxed around you, but he is certainly careful of what he says, just in case it could offend you in some way. He would never want to offend you in any way, darling.

Sometimes he would cook for you, which was not unusual for him, but cooking for you just felt so different. He put so much care and effort into the dishes he made for you, being sure to only include things you liked and weren't allergic to. Hm? You're a vegan? Well darling, Alastor makes excellent vegan dishes just for you!

The biggest sign that Alastor is in love with you is that he allows you to touch him, whether it be through hugs that you initiate or him letting you touch his ears.

The only people who might be able to notice the subtle differences would be Husk or Mimzy, both of whom know Alastor well. It would most likely be Husk who would notice the affection he shows you.