The lack of standards falls on the acting part. I ran a quick search and found that SWGDE best practices guides and documents do consider the case for the presence of malware on the digital evidence sources on many different scenarios [1]. Having an "expert" who is unaware of these guides is another story.
Do you have anything specific you're pointing to in those search results? Reading the excerpts, all but two are talking about malware on the analysis machine.
2012-09-13 SWGDE Model SOP for Computer Forensics V3-0 merely says to detect "Detect malware programs or artifacts".
2020-09-17 SWGDE Best Practices for Mobile Device Forensic Analysis_v1.0 seemed the most in depth, and it merely states:
> 9.4. Malware Detection Malicious software may exist on a mobile device which can be designed to obtain user credentials and information, promote advertisements and phishing links, remote access, collect ransom, and solicit unwanted network traffic. Forensic tools are not always equipped with antivirus and anti-malware to automatically detect malicious applets on a device. If the tools do have such capability, they do not typically run against an extraction without examiner interaction. If the examiner’s tools do not have antivirus/anti-malware capability, the examiner may need to manually detect malware through the use of common anti-virus software applications as well as signature, specification and behavioral-based analysis.
No, I just went to search if the topic is mentioned in guidelines (which it is, multiple times). I'd then expect a (good) expert to pick on those breadcrumbs and search on how to do that (if they don't have the skills already). If I were working on a computer, I'd try to find IOCs that point to an infection (or lack of evidence for it).
If there's a memory dump to work on, a more in-depth analysis can be done with Volatility on running processes, but it usually falls back on the expert having good skills on that kind of search (malfind tends to drop a lot of false positives).
But at least the guides gave a baseline/starting point that seems to be better than what was described. It's very difficult to prove a negative, so I'd also be careful with the wording, eg: "evidence of a malware infection was not found with these methods" instead of "there's no malware here".
What I quoted perfectly describes what they did. Ran one off the shelf antivirus scan and then considered the concern addressed.
It's obviously impossible to disprove a system had malware on it, but that fact itself should be part of any expert testimony. Especially testimony for the defense in a criminal trial.
[1] https://www.swgde.org/?swp_form%5Bform_id%5D=1&swps=malware