I lost my phone and while using the find my phone application, it would show thatmy device is offline....someone tried to log into it but since the passcode was send to my other phone, they could not access it. Now, find my iphone shows no device

If you got a request for 2 factor auth code then yes someone knows your apple ID and password. But you rejected the request hopefully??

Did you respond to a text saying your phone has been found? if so it was a scam and they got your details.

Change your apple id password now!

Change your Apple ID password - Apple Support

You only get a 2FA code request if they know your apple ID password. If someone just tried to break the passcode or password you would never know.

Learn more about how use the Find My App\

Find My - Official Apple Support

Then look at this Apple article and my additional advice below it:

If your iPhone, iPad or iPod touch is lost or stolen – Apple Support (UK)

Tracking lost or stolen phones

The 'Find My' tracking is to help you discover if you left your phone in the office or the restaurant. Lost or Stolen phones are rarely ever seen again.

Risk mitigation

Get your provider to

Cancel your SIM.

Send you a replacement SIM on same number.

You can then put the SIM in another iPhone to get the codes you will need.

Phishing warning

Its possible for a thief to obtain your phone number particularly if you did not have a PIN set on the SIM. So do not respond to texts to sign into your account! They are NOT from Apple.

Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support

Resign to the fact that if you had a passcode set and you do not provide your Apple ID via phishing texts then your data is safe but you will never see the phone again.

by the way, you say the code was sent to your "other phone."

If you said OK when the request came in, the code went to ALL your devices including the lost one in which case they got in, probably turned off Find My on the phone and sold it Hopefully that is all they did because without the passcode they would have to wipe it.

Change your password as in my last post!

Yes it may just be lying in a drain somewhere and died.

The request for a 2FA code still concerns me so don't forget to change your password.

If you are now on a different phone number then the scam of removing the SIM to get the number then texting a week later is less likely.

my se was lost, then I received a email saying someone maybe I was trying to log in to it, then it disappeared as a listed device

See all of my post above yours.