Rowhammer and similar bugs where the attacker can cause bitflips but not have tight selection of which bits are flipped are going to leave evidence.
On systems with ECC, you're highly likely to get elevated correctable and uncorrectable error counts. Most systems will halt or reboot on uncorrectable errors, and usually that leaves a lot of logs. Correctable errors should also leave a trail, too. With RowHammer, sometimes you get enough bitflips that ECC can't detect or correct the error, but it's a distribution and many of the bitflips will be detected or corrected. I don't quite understand this one, but I imagine it's going to be similar --- you usually need to do a lot of attempts to get the bits you wanted flipped, and some of those failed attempts will trigger ECC; if the attacker gets really lucky, there won't be a trace, but if they're attacking multiple systems, they're unlikely to get the flips they want on the first try on all the systems.
On systems without ECC, you're likely to get increased crashiness on systems under attack. It'll be much harder to track down of course, but it won't be without evidence.
Speculative execution attacks to read memory can leave evidence too, but it depends on what level of statistics are tracked. I would suspect few systems collect the detailed statistics you'd need.