2012 Mazda 5 with 100k. Opened the hood and discovered brown liquid, which looks yellow when it’s dried, splattered all over the passenger side of the engine. Doesn’t smell like oil…maybe brake fluid? No obvious leaks or source when the engine is running, and no other symptoms when driving so far. Ideas?
Black - oil. Check level, fill, get fixed when convenient or put a rug under the car to catch the oil and just top 'er off when you get gas.
Red - tranny fluid. Check level, fill, schedule service or plan an afternoon to track it down asap
Green - coolant. Fill up, keep an extra gallon in the car, pick up a new thermostat on your next trip out.
Clear - water…though if there’s no reason for it (no A/C running) check by hand to make sure it really is water!
Brown - Probably power steering fluid.
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The pic shows the pass side area above frame rail with coolant tank. Coolant can make a brownish stain when it sorta dries on a dusty/dirty area. How is your coolant level?
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Coolant comes in a multitude of colors these days; Green, red, orange, yellow, pink, and blue.
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Regardless of the color coolant comes in, when it is past fully depleted, it turns brown. That is severe neglect of the cooling system and a full system drain and fill needs to be done immediately if not sooner.
Edit: It could also be oil that has mixed with or absorbed some water. This does not mean that there is a problem other than a dirty engine and maybe a very minor oil leak. If the oil on the dipstick looks OK, no issue.
Could it be a leaking engine mount? 
I’m guessing it’s coolant. It can turn brown if it is left in the engine long enough to break down and cause rust. When is the last time you changed the coolant?
I can’t tell of you’re joking, so I’ll answer: No.
That could be a hydrostatic engine mount and it seems that the fluid is mainly in that area. Hydraulic fluid from an engine mount can appear quite dirty.
Coolant is totally full and bright green/blue.
It might be a leaking engine mount. Mazda does use a fluid filled mount in this car. See link to Rockauto below. A fluid filled mount is typically filled with coolant so the color would be green-ish.
https://www.rockauto.com/en/moreinfo.php?pk=8104032&cc=1501076&jsn=382