Why do my emails disappear sometimes? Earlier this year, I had to email a colleague to ask them to send me some information I was waiting for. They insisted they'd sent it already but I couldn't find the email in Mail.app (both iOS and Mac) or Gmail.

The same thing happened today. A different colleague emailed me over the weekend. I saw the message arrive in my inbox on Saturday but didn't open it. This morning, I couldn't find it so I emailed the colleague asking them to send the info believing I might have imagined it the first time. The colleage replied. After reading the reply on my phone, I went to my mac, opened Mail and the message wasn't there! I couldn't find it on my phone or the Gmail web app either. I looked in junk and trash as well but still couldn't find it.

I've tried rebuilding the mailbox but the missing messgae is still missing.

It's a bit worrying as I do all my business over email. Why do my emails disappear sometimes? What can I do to prevent them from disappearing?

(From time to time, I hear the incoming mail alert but there's no new email in my inbox. Could that be related?)
It isn't in the nature of email itself for emails to go missing.

This means that mails go missing either because:

(a) someone is accessing your account and deleting the mail,

(b) there's a server problem where data is lost,

or

(c) software has been setup to intentionally delete emails

You indicate that you're using Gmail, but also Mail.app on iOS and Mac. If you store your email on Gmail exclusively, it doesn't sound likely that (b) happens several times for you.

If your account is secure and you haven't shared your password with anyone else, I don't think the problem is related to (a) either.

So the most likely culprit is reason (c). This could be a spam filter, antivirus/antimalware software, etc. that is setup to identify and delete suspicious emails. As you're using multiple clients to access mail, this could have been setup directly on Gmail, in Mail.app on your Mac or on an iOS client.