I was just going over some configuration on the Shopify store when I ran into this
<img src='https://sl-promoserver.herokuapp.com/conversion?order={{order_number}}&shop_domain={{shop.permanent_domain}}&subtotal_price={{subtotal_price}}&total_price={{total_price}}&code={{discounts.first.code}}&line_items={% for item in line_items %}{{item.quantity}},{% endfor %}' width='1' height='1'></img>
It sits in Additional Content & Scripts box in Checkout configuration. It's some kind of analytics service but I don't think that it's either sumAll or MarketBrain that I'm using. Definitely should've commented this.
Does anyone know what runs on sl-promoserver.herokuapp.com? The name makes me wonder if this was to be just a temporary mechanism for collecting data. I wonder if it does indeed belong to sumAll or MarketBrain.