Wikitionary states that it is used in the dialects of Najd and Iraq. Originally, it was used to mean "unexpected guests". However, in the current usage, it means just "guests".

في اللهجة السعودية (نجدية) واللهجة العراقية: جَمْعُ تَكْسِيرٍ مُفْرَدُهُ خَاطِر بمعنى الزُوَّار والضُيُوف. أصلها للذين يأتون فجأة بلا موعد، ثم غلب استخدامها على الجميع.

https://ar.wiktionary.org/wiki/خُطَّار

In another website- that I do question its entries many times- this word is listed under hassaniyya dialect, used by Mauritanians and Sahrawis.
https://ar.mo3jam.com/term/خاطر#Iraqi

If this is true, I believe that this word has a Fusha grounds being used in three distant parts in the Arab world, Najd, Iraq and Mauritania.

Looking it up in classical lexicons, I have not found senses to do with guests being unexpected or not. Anyhow, the closest sense, that I could find, is
خَطَرَ بِبَالِي كَذَا خَطْرًا، وَذَلِكَ أَنْ يَمُرَّ بِقَلْبِهِ بِسُرْعَةٍ لَا لُبْثَ فِيهَا وَلَا بُطْءَ
[Ibn Faris]
According to that sense, a similarity might be drawn between a passing thought and the passing wayfarer who stays for a while and then departs (an unexpected guest).

HTH