Ha Robin was posting while i typed ... it think he touched on what im saying here ...
its too hard to say without seeing , it could be genuine ... like pick a really bad patch , give it lots product and a hell of a going over just to show a difference for the picture .... or it could be a visual trick ... wet the carpet to make ot look dark , the cleaned patch will stand out more , it could be a creative camera angle or even some sort of foam like u suggested .
regarding resoiling .... the origins of whole detergent free movement is now so mixed up its hard to tell what or who its suppose to benefit ,the reasons for its existince seem to swing between , care for the carpet , envionmental concerns , health of the user or customer concerns ... its gathered these as marketing interests latch onto the latest public concern fads .
The origin as i see it is somethimng like ... started in germany , guy makes soap free cleaning solution .... soap at the time was linked to resoiling carpet ( soap is not used in any carpet cleaning product now though )
The product finds uk investers and somehow takes on the name microsplitter ( we now know this product was a just a basic component used in many detergents ie phosphate )
microsplitter catches on amongst the uk carpet cleaning community , who are eager to buy into the idea of using a ' new technology ' and addition claims become accepted , like it dosnt need heat or that it ties in with the whole eco sales strategy ( none of which really stack up )
Other carpet cleaning chemical suppliers now latch onto this market and its takes on the collective name ' detergent free '
most contain sodium carbonate or phosphate , basic detergent ingredients ... there is no new technology
So from the original product which omited soap .... as soap is now no longer a component used in modern carpet cleaning .... they seen to have latched onto the modern synthetic surfactants as the ingredient to be left out ....
so if you take a product like ultimate master ... which would be class as a detergent ... a detergent free product would be just like ultimate master ... as in sodium carbonate ,phosphate , ... but they would leave out the ethoxylates and small bit of citrus solvent
But why .... where is there any proof that these modern ingredients cause either resoiling or health or environmental issues ....none ... marketing has created the detergent free sector
So to answer you question .... no most good carpet cleaning detergent products will not cause unusual resoiling if used correctly either intank or prespray .
I get do some plush places ... i almost always have ultimate master in the tank beacuse i see a benefit in th eresult , if there was unusual resoiling i would see it in the repeat customers carpets .... or i would not get back in the first place .