We've been messaging back and forth and have similar ideas about ppm vs. monthly. I suggested we meet for coffee, and she replied, "I'd prefer to meet in. A more discreet place. Is your place okay?". Is this a red glad? It feels weird to me.

some ladies dont want to be seen in in public with an old guy. The big downside for you is if she is a fatty which is not uncommon.

Not really uncommon.  I get lots of POT SB's who are probably pro or semi pro who just want to meet at my place to fuck and leave.  Most of the time I pass because I prefer the GND/GFE type of SB more than the UTR pro. If you want to proceed and you have negotiated the sugar and specifics you want during BCD, then go for it.  

BUT - if it's the first time you've met take some reasonable precautions:  

1. Google her number to look for info about her.  If you get any hits make sure the info checks with her story. (i.e.: No escort ads, no arrest info, etc., phone area code is from the area where she lives or lived.)  
2. If you can get her real name, look her up on Facebook and LinkedIn.  Offer to swap pics of your drivers license if you feel the trust is there.  She's taking risk coming to you, and you are taking risk letting a stranger into your home  
3. She comes alone, or by Uber. No drivers or friends who will wait outside.  
4. Hide/lock up your wallet, credit cards, cash, etc.,  except for her sugar.  You don't know if she will get sticky fingers while you go to the bathroom.  
5. Get a clear face pic and send her one of you.  Check before you open the door.  Fake pics = danger!  
6. If you are a little leery of her "honesty" and intent, demand a skype or Face Time chat to let you get a feel for who she is.  

There's probably several more precautions you can take, and fellow posters may add them, but you get the idea.  You  probably don't want her 450 pound coked-up, shotgun toting pimp busting down your door and robbing you of everything you have.

Definitely not a red flag, and very common.  BUT...it should cause you to think about the type of experience you seek.  Meeting out is a screening tool.  If you are looking to increase the likelihood that you are a good fit, meeting out gives you the chance to build/assess connection.  If you meet at your home, what are the chances you are going to send her away if the vibe is a little off?  If you are a typical guy, like most of us here, you won't.   That being said, if the vibe/connection is there during the messaging process, it is likely that you will have a nice time BCD.  But in your post, you say it feels weird to you...weird enough that you are going to feel awkward, or weird because it seems "too easy"???   If it is the latter, go for it...she is easy...

is to enter her phone number in the facebook search box. If her number is linked to her fb account you can find out lots about her. I've used this technique many times.