As the saying goes, one thing leads to another. I recently had a request to flag any access to a SharePoint folder and alert on it. There are many ways (as I have found) to accomplish this. Depending on who wants to be alerted, and what kind of licensing you have in Office 365, will depend on the path you take.
Audit Log Search
The easiest way to do this (albite being very broad, and not very customizable), is using the Audit log curtesy of Microsoft Auditing Service. To set this up, just simply follow the instructions here to setup the "Admin Role" to view the audit logs (please don't ask why this is in Exchange).
Once that is done, you can create an audit retention policy in the Compliance Centre (at the time of writing 😜) to retain the logs for however many days you need (based on your compliance policies).
Then use the search option to search through the logs as required...
Cloud App Security
Another way to do this is Microsoft CAS (Cloud App Security). You may create an Activity Filter and set an alert from the GUI which will flag any such access like the below.
However, if you want to be more discrete, you can use the MCAS API access method to run a PowerShell script that will do this for you either regularly or on-demand.
Note: Before you can run the script, you will need to install the MCAS module, create a CAS API Token, and save the provided secret key and URL to be used by the script.
Download v.0.1 - 2017-09-13 -get-McasFileAccess.ps1