Ads appear on the site, although the code is clean?

Question

Malware

Hello!
Guys, please help, maybe someone knows.
The essence of the question is that I wrote a service for the customer, the code is completely clean, the customer sends screenshots with the question - what kind of advertising did you put me in?
The main thing is that when you refresh the page, it disappears, then after a while it appears somewhere, just some unpredictable crap.
Checked the code, everything is clean. Does anyone have any experience with this hat? Help me please.

Answer 1

0) [not mine] The Internet provider is playing around (see the topic about the megaphone)
my options
1) some of the banner / advertising / link exchange sites were scorched, draining
mobile traffic on weekends. Those. Are there external js scripts on the site? Perhaps one of them screwed up.
2) still look in the database (more likely for ready-made CMS).
SQL vulnerability , execution of code from the download folder .
The server was hacked, your password was leaked.
3) I know how to modify the page through Apache and nginx configs.
4) Inject into temporary/cached php files