Bluntness
Micro-oganisms tend to need quite specialist conditions for growth
As a rule of thumb keep food hot or cold and you will control most pathogens, bit there are other barriers or hurdles that can be used alone or in combination to control them. Salt, sugar, acid, reducing water content, removing oxygen availability, chemical preservatives etc.
That's why something like a salami can be kept at room temperature even though it is raw meat - because it's fermented so acid is produced, dried, so available water reduced and salted.
Some organisms have evolved to cope with hostile environments though and can survive and multiply in conditions that would inhibit or kill most others.
In our sausage example, we have OP's description of bulging packaging and fizzing. Fizzing implies fermentation, so most likely lactic fermentation as sausages are prone to this, or possibly a yeast. Either way we have either acid or alcohol being produced, both of which will inhibit most bacteria. Sausages are also usually high salt and may have some spices, many of which also have natural anti-microbial properties. Also often nitrates, nitrites or other preservatives.
The bulging also implys packed in some sort of modified atmosphere packaging ( oxygen replaced with other gases), which would reduce or prevent growth of a lot of pathogens.
In a reduced oxygen atmosphere, you have to worry about your anearobic spore formers though, many of which are pretty nasty. Think of them as a seed or egg which will "hatch" producing toxins as they do so. Both spores and toxins are heat stable, so not destroyed by normal cooking and don't need oxygen to survive. But the additional salt, acid etc will help reduce the risk. ( This is why low acid canned foods need retort processing, but something acid like tomatoes need a much less aggressive thermal process).
So even though sausages were at room temperature in the bin, the reduced oxygen, increases acid / alcohol and high salt / preservatives would still control anything too sinister going on.
OP's DH then scavanges them from the bin and cooks them, killing off our friendly lactobacillus / yeast, but having inadvertently produced a pH controlled cooked meat product, that, provided he doesn't come up with any other bright ideas, like using Tupperware that was storing raw chicken as his lunchbox without washing it, should be stable enough for a trip to the office and back, particularly if he put it in the fridge while he was there.
He made a lucky choice when selecting his bin meal, but it goes without saying, don't try this at home!