Found in file \woocommerce-appointments\includes\integrations\woocommerce-product-addons\includes\class-wc-product-addons-cart.php on line 669.
\woocommerce-appointments\includes\integrations\woocommerce-product-addons\includes\class-wc-product-addons-cart.php
Some people want to remove the plus sign from addons but still need to keep the price.
Found here.