Ok so this happened tonight and I thought it's no brainer - dead battery. Then I've googled and stumbled upon "Economy Mode" which should be triggered prior to the battery issues. I was driving a car for about 20-25 minutes and parked to get groceries (pause was maybe 5 minutes max). So I turned the ignition, waited a few secs for bulbs to go off and it started cranking slowly going slower, dimming all the displays and after 2-3 seconds it went dead. Trying again - same issue. I made a pause for a minute or so, tried again and it started normally. Got home (10-minute ride top), turned the engine off - tried to start it right away twice, dead again (slow cranking than dead).
Maybe relevant: In the past two months it happened twice that car is cranking normally but not starting. No errors are shown on the dash or diagnostics. It would take me 3-4 times in a row to start it. After a successful start, it was all good. So only twice in two months and the car is driven daily.
P.S. Also I had issues with turbo when accelerating hard around 3000 RPM in third gear - DPF error with error code are thrown and car aggressively brake (like engine break). Went into Citroen dealership - we are waiting for turbo copy sensor - I don't see how this is related to previous issues since starting is not turbo related?
Citroen C4 Coupe 2.0HDi VTS (136HP) 6 gear manual 2007
UPDATE: I've visited shop today and it's definitely not a battery. Initially, they thought it's an issue with the contact as my + connector was a bit loose. When cranking slowly that + connector gets really hot and there's a bit of smoke too. Looks like something is drawing more current than it should so I'm waiting for an update tomorrow regarding starter...