I would assume it depends on two factors:
- exact circumstances
- the sources you go by
The latter is GW's usual carte blanche that would declare any answer to your question viable, whereas the former would refer to the nature of the mission, the result of the battle, and how much time the troops have to scour the battlefield for recovery ops. The Space Marines' lightning strike tactics are an important aspect of their background, so spending too much time surveying the battlefield might not seem fitting.
Perhaps the wargear is important, but for the most parts still deemed replacable. Meaning, the outside world might lump Marine gear all in one category of importance, but within a Chapter there are "degrees of replaceability" influencing the effort made to their recovery.
Given that Space Marines are based on RL feudal knights, and how valuable platemail armour was in medieval times, how did that work? Do we have sources on that subject?
I have read about some Chapters going through considerable pain to recover their equipment, although that particular instance refers to Rhino APCs rather than "smaller" gear such as armour and weapons.
"Should a Rhino ever be lost in battle it is an occasion of great mourning for those entrusted with its care, and furious battles have been fought to reclaim the burned-out carcass of a Rhino simply to lay its spirit to rest. After the Battle of Naeuysk Gorge, fourteen Rhinos of the Imperial Fists had to be abandoned when traitor Space Marines from the Night Lords Legion ambushed the advancing column as it crossed the only bridge across the gorge. […] The following morning a daring mission involving an airborne assault across the gorge pushed back the Night Lords from the hills on the opposite side and allowed the Imperial Fists to bring up salvage units to start the recovery of their shattered vehicles and the bodies of their comrades. The battle in the hills raged for over thirty hours, with Imperial Fists' casualties amounting to almost 85% as they fought to give their Techmarines enough time to retrieve the fallen Rhinos. Many of the recovered vehicles were subsequently repaired and sent back into action, their battle spirits eager to avenge the ignominy of their earlier defeat."
- WD #269
Note the mention of salvage units. Perhaps another use for Chapter Serfs, too?
As far as Chaos looting stuff from loyalist Chapters is concerned, I did read about them doing this a lot, and in fact this being their primary means of restocking their depleted armouries. Given that they no longer have the support of the Adeptus Mechanicus, or a Chapter Homeworld with a proper Forge, they'd need to rely on looting Imperial armouries or doing shady (and likely fairly rip-off) deals with the independent "Dark Mechanicus". Some of the larger warbands might have re-established a modicum of production capabilities on conquered worlds using slave labour, although the chief limitation is the likely lack of technical knowledge.
Loyalists looting Chaos gear, on the other hand? Tricky. Might depend on the Chapter, but I would assume there's a high chance it would be deemed irrevocably tainted and destroyed.