He knew about the engagement ring

 (Sherlock meta by audreyagnes2)

There’s a haunting question among the sherlolly shippers: [Did] Sherlock know about the engagement ring before he mentioned its existence at the train guy's flat? 

I think yes, he knew it and I bet it was when he went to St. Barts after his reunion with John.

There are a couple of moments that can confirm my theory, but the only one that it’s in the same episode is the brief moment with the host in TEH.

 Remember when Sherlock stepped in to the restaurant and he barely saw the host? At [that] exact moment the words “expectant father” appeared on his shoulders and then he received a phone call.

In conclusion he actually observed, even if it was just a few seconds of sideways. (Nothing that you didin’t know before). 

My theory is, at the exact moment she entered in the lockers room, he could see the amount of gained weight she had (Sherlock pointed [out at] different [occations] to Molly and John, the amount of weight they gained by domestic bliss), and other things she added in to her life during his absence, including the engagement ring.

Why isn’t it showed at the moment? Well, the show needs the surprise factor (a regular basis in the show), but in this season we can see how much Sherlock learnt to be kinder with his friends and with Molly.

So, the look he gave to her when she asked “have dinner?” it’s a look of restriction and now fits perfectly.

“Molly, you’re engaged, you have a fiancé to think about before going out with me the love of your life”. 

You will ask: but what was the flirty look they shared in the train guy's flat? Sherlock was flirting with someone who was engaged? Remember: forbidden is tempting.

Maybe it was the next kick he needed to reach another step [towards] Molly, because he asked at the end of their day to go for fish and chips, and if she wouldn’t have asked him about [why he wanted to] work with her, we don’t know if they actually would’ve been eating fish and chips, because that talk lead to the engagement ring thing, and how wrong [it] was [for him to] ask her out for dinner, and obviously how dreadfully wrong she was accepting it, because they actually enjoyed their day working together more than they should. 

Maybe you will ask again: but, if he saw her ring before [he] asked her [to be his] crime solving companion, was [it not equally] wrong [for him to] ask her to go for Fish and Chips? Not necessarily. 

We saw his expression when she arrived to his flat and before he had turned around, [we saw] the face of someone who was aware of something, maybe nervous, but when he faced her, it [was] someone who *thought* he had everything in control.

Again, fits perfectly.

Maybe he was in control at that time or he thought he could control himself, but for sure he wasn’t expecting [to] enjoy the day with Molly.