well, fancybox.png is to big to be inlined (it's 14Kb, AO by default only inlines if < 4Kb). but if you have "data:uri's" active, if you have a CDN specified and if the image is too big, then the url_replace_cdn-function is called and with that also the "autoptimize_filter_base_replace_cdn"-filter. so if those conditions are met, the background-image URL should get swapped for the one in your function hooked to the filter, really ...
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