Is she interested in more than friendship? I can't tell.

[OldDogNewTricks]

I am confused as what to think/do.  Just to let you know my background, I work as a contractor and visit a particular site once a month.  I used to work in a separate room but for the past couple of years set myself up in admin office.  There are 2 that work in there but when I visit there is only 1 person in and I work on the empty desk.  I shall call her Sarah.  I talk to Sarah quite a bit while I work in there and have really got on well.  We talk about lots of things and discuss a lot of things like politics, personal interests etc.  She has always sent me a jobs list by email a couple of days before my visit so I know what I am expected to do while there.  Anyway, for the past few months, when I am due to visit, she always sends me an email but always adds at the bottom that she is looking forward to seeing me.  I never gave it much thought but it now seems to be every time.  She is now leaving her job to take early retirement as she is in her late 50s (I am late 40s).  When I found out she was retiring, I feigned sadness and I playfully said to her I will miss her and can’t believe she was leaving me.  She has now suggested that she would like me to visit her at her home for a coffee and a chat when I go the place I work at as she lives a couple of doors down the road.  I am not sure what to think.  I hope she wants to take this a step further but I am afraid I may be misreading the signs and she is just happy in my company.  She is married and I am involved in a relationship too.  I know this is probably wrong but I find more in her than I do my current partner.  If she wants to have a bit of fun on the side I wouldn’t be against that.  I just find it very odd that she would invite me into her home just for a coffee and chat.  I do find her attractive and if she made a move I would definitely respond in kind but worried if I make a move I will look like an idiot and she may report me to the company I work for.   What should I think?