First, it's important to understand what Malwarebytes Anti-Malware for Mac does. It only looks for installed components of malware, adware and PUPs (Potentially Unwanted Programs). This also means that it only searches the current system drive, and doesn't look at external hard drives. There are many ways that you could have something malicious on a hard drive without it actually being a threat.
In addition, it only looks for Mac threats, and only threats that are recent enough to be likely to be found on a Mac running OS X 10.7 or later. So, very old and inactive malware or Windows malware are not detected, because neither is a threat to your Mac.
Finally, it does not examine every file on your hard drive. Exactly where it looks is defined by each rule in the signatures database. Each rule looks for a specific pattern in one or more specific places. This allows scans to be very fast, and allows us to focus on solving the user's problems.
In light of this, there are a number of possible reasons that other anti-virus software may find things that Malwarebytes Anti-Malware for Mac does not.
- It's Windows malware.
- Windows malware may be attached to an e-mail message, for example. Never allow anti-virus software to remove malware from Mail's mailboxes, as doing so can corrupt those mailboxes.
- It's on an external hard drive.
- For example, malware could be in your Time Machine backups. Never allow anti-virus software to remove malware from Time Machine, as doing so can corrupt your entire set of backups.
- It's not installed.
- There may be pieces of malware on your hard drive that are not installed and cannot function by themselves without being installed.
- It's a false positive.
- All anti-virus software is susceptible to the possibility of falsely detecting a legitimate file as malicious, some more than others.
- We missed it!
- Of course, detection of malware is a constantly-evolving thing, so there are always going to be new things that we miss. If you believe this to be the case, open Malwarebytes Anti-Malware for Mac, then choose Contact Support from the Help menu. Then follow the instructions to send a request for help, along with a "system snapshot" (a profile of your system that can help us diagnose new issues).