This Kanji is handwriting shortcut for”開”.  It’s called 略字、崩し字、etc. 略字 is less common in public places because it’s not formal. Japanese people (especially elderly) often use 略字 in private notes like a memo, diary, handout, etc.

Here is an article about 略字. I think some of them are very unusual even for us.

https://soranews24.com/2017/02/02/w-t-f-japan-top-5-most-ridiculous-kanji-handwriting-shortcuts%E3%80%90weird-top-five%E3%80%91/

ミイラの飼い方

開けただけだけど力尽きた！

きっと このミイラは小さいので、ドアを開けるだけでも大変なことなんですね。